1. Field of the Invention
This invention relates in general to structural members and methods of forming structural members. More particularly, but not by way of limitation, it relates to composite, pre-stressed structural members and methods and apparatus for forming, designing and pre-stressing such structural members.
2. Description of the Prior Art
In the field of constructing composite, pre-stressed structural members, many methods of pre-stressing are available. A particularly desirable method of pre-stressing such composite structural members is shown in my U.S. Pat. No. 4,493,177 and my U.S. Patent Application Ser. No. 688,272, filed Jan. 2, 1985. The pre-stressing disclosed in this patent and patent application is achieved by forming the composite structure upside down. The upside down forming includes connecting the steel beams of the composite member to the upper side of a mold so that shear connectors extend downwardly into the mold. The steel beams and the mold are joined and supported so that deflection of the mold causes a parallel deflection of the steel beams. As the mold is filled with concrete, the steel beams and mold deflect downwardly from the weight of the beams, mold and concrete, thus pre-stressing the beams. The top flange of the inverted beams (bottom flange when upright) receives a compression pre-stress. After the concrete hardens, the mold is removed and the connected beams and concrete slab are inverted so that the composite structure is upright. In the upright position the bottom flange of the beams receives a tension stress which is reduced by the compression pre-stress achieved by the inverted molding. The concrete, of course, receives a compression stress.
This type of pre-stressing is especially desirable because it produces an improved pre-stress resulting from the pouring of the concrete itself. No separate pre-stress activity is required. In addition, because the uppermost or surface concrete is the concrete formed at the bottom of the mold, the concrete surface is less permeable and harder than concrete structures which are not inverted. Still further, this type of pre-stressing results in a pre-stress relationship based upon the weight distribution of the concrete and beam combination. This pre-stress relationship is much improved compared to pre-stressing resulting from jacks which concentrates more of the pre-stressing at a single point.
Despite the great improvements over the structures and methods of the prior art, my method and apparatus can be further improved by the invention described herein. Particularly, the present invention provides improved strength and resistance to bending with less cost.